The utilization of coal in the prior art has been minimized due to undesirable emissions, such as oxides of nitrogen and sulfur, particulate emissions and greenhouse gases such as carbon dioxide. As a result, there have been efforts to reduce these emissions and improve fuel efficiency of coal plants.
One of the systems that have been developed is the Integrated Gasification Combined Cycle (IGCC) system for use in power generation. IGCC systems were devised as a way to use coal as the fuel source for a gas turbine plant. IGCC systems are clean and generally more efficient than conventional coal plants.
IGCC is a combination of two systems. The first system is coal gasification, which uses coal to create a clean-burning synthetic gas (“syngas”). The gasification portion of the IGCC plant produces syngas, which may then be used to fuel a combustion turbine. In the gasifier coal is combined with oxygen to produce syngas, hydrogen and carbon monoxide. The syngas may then be cleaned by a gas cleanup process. After cleaning, the syngas may be used in the combustion turbine to produce electricity.
The second system is a combined-cycle, or power cycle, which is an efficient method of producing electricity commercially. A combined cycle includes a combustion turbine/generator, a heat recovery steam generator (HRSG), and a steam turbine/generator. The exhaust heat from the combustion turbine may be recovered in the HRSG to produce steam. This steam then passes through a steam turbine to power another generator, which produces more electricity. A combined cycle is generally more efficient than conventional power generating systems because it re-uses waste heat to produce more electricity.
IGCC systems offer several advantages over current conventional coal-based power generation systems. One advantage is reduced emissions. Another aspect of IGCC plants is that emissions clean-up, including removal of sulfur and carbon dioxide, may be effected upstream of the combustor system in the fuel stream. Since this stream is far smaller than the entire flue stream, emissions removal equipment for an IGCC plant are smaller and require less capital expenditure than a conventional coal plant of like output.
IGCC systems offer other advantages, such as higher efficiency, less coal used, higher turbine outputs, and the production of additional chemical by-products, such as hydrogen, which may be used as an alternative source of energy in other developing technologies.
Nevertheless, IGCC systems still suffer from some disadvantages when compared to other systems. For instance, since syngas has a significantly lower heating value than other fuels, IGCC systems require more syngas to produce a selected turbine operating temperature.
Another disadvantage is that IGCC plants are normally designed to operate at a design operation point for output and efficiency. Accordingly, plant systems, components, and controls are structured to meet a design operation point for fuel stream temperature and heating value that deliver the design operation point output and efficiency. This lack of flexibility causes conventional IGCC power plants to reduce output, i.e. part load the gas turbine, by partially closing the Inlet Guide Vanes (IGVs), reducing the firing temperature of the combustor, or a combination of both. Both of these control mechanisms result in a reduction of IGCC efficiency at power output levels less than the design operation point.
Accordingly, it would be beneficial to provide a coal-based power generation system that has increased efficiencies as compared to prior art systems. It would also be beneficial to increase the operating capabilities of the IGCC to increase operational flexibility for IGCC gas turbine systems. It would also be helpful to provide an IGCC plant that offers increased efficiency at reduced output levels and provides improved operational flexibility compared to prior art systems.